Competition between entropy and electrostatic interactions in a binary colloidal mixture of spheres and platelets

We describe the phase behavior of an aqueous mixture of discotic nanoparticles of laponite and spherical magnetic nanoparticles of maghemite. To obtain stable mixtures from a chemical point of view, the maghemite nanoparticles are first coated by a thin layer of silica in order to adapt their surface chemistry to that of laponite nanoparticles: this enables one to raise volume fractions of maghemite Phi mag in the laponite suspensions up to several percent. Although the system is out of equilibrium, a "fluid-solid" state diagram was established showing that the mixtures undergo a fluid-solid transition, similar to that of pure suspensions of laponite, over a given volume fraction of laponite Phi lap and over a given Phi mag. An increase in Phi mag shifts Phi lap toward the lower values. When a solid sample is just above Phi lap, the application of an external magnetic field gradient induces a solid-to-liquid transition if the sample is located not too far from Phi lap on the state diagram. The structure of the mixtures, determined either at small scale by small-angle neutron scattering (SANS) or at intermediate scales by optical microscopy, shows that the solid samples are phase separated at a local scale: they are made of densely connected domains of laponite nanoparticles surrounding liquid pockets of maghemite nanoparticles. The size of the pockets grows with time. The magnetic liquid pockets are responsible for the rupture of the solid samples when an external magnetic field gradient is applied since their deformation induces local mechanical stress, internally damaging the network formed by the solid domains of laponite. The microscopic phase separation is the result of two opposite effects: (i) entropic effects that tend to phase separate the system macroscopically when the packing entropy overcomes the orientational entropy and (ii) long-range electrostatic repulsions that freeze the system.     

Triplet formation involving a polar transition state in a well-defined intramolecular perylenediimide dimeric aggregate

A cofacially stacked perylenediimide (PDI) dimer with a xanthene linker was studied under a variety of conditions (solvent, temperature) and serves as a model for the molecular interactions occurring in solid films. Intrinsically, the PDI units have a fluorescence quantum yield (Phi F) close to unity, but Phi F is lowered by a factor of 6-50 at room temperature when two PDI moieties are held in a cofacial arrangement, while the decay time of the most emissive state is increased significantly (tau F = 27 ns in toluene) compared to a monomeric PDI molecule (tau F = 4 ns). Fluorescence measurements show a strong solvent and temperature dependence of the characteristics of the emissive excited state. In a glassy matrix of toluene (TOL) or 2-methyltetrahydrofuran (2-MeTHF), Phi F is high, and the decay time is long (tau F = approximately 50 ns). At higher temperature, both Phi F and tau F are reduced. Interestingly, at room temperature, Phi F and tau F are also reduced with increasing solvent polarity, revealing the presence of a polar transition state. Photoinduced absorption of the stacked molecules from the picosecond to the microsecond time scale shows that after photoexcitation reorganization occurs in the first nanoseconds, followed by intersystem crossing (ISC), producing the triplet excited state. Using singlet oxygen ( (1)Delta g) luminescence as a probe, a triplet quantum yield (Phi T) greater than 50% was obtained in air-saturated 2-Me-THF. Triplet formation is exceptional for PDI chromophores, and the enhanced ISC is explained by a decay involving a highly polar transition state.     

Simulation of gas diffusion in porous layers of varying structure

Gas diffusion in porous layers of varying structure was simulated numerically. Mesoporous mesophase material (MMM) and silica gel layers were studied. The former were a set of ordered cylinders; the latter were disordered packings of spheres. The average residence time of a molecule in a layer (return time) and dispersion of this time in relation to the layer depth were calculated. For the same porosity and specific surface of layers, the average return time is independent of the pore structure and increases with the layer depth as a linear function. This is the consequence of the general theoretical result, according to which the duration of molecule wandering in a pore depends only on the ratio of the pore volume to the section area of its windows. Dispersion of the wandering time is sensitive to the pore structure; it is slightly smaller for regular pores than for a complex pore system. The functional dependence of return time dispersion on the layer depth is the same for different layers (the cubic root of dispersion changes with the layer depth as a linear function). This work helps us to understand recent experimental data, which showed that using MMM for gas chromatographic columns increased the efficiency of the latter compared with other columns based on silicon oxide.     

Ab initio calculation of the electronic structures of the 3Phi ground and 5Phi excited states of CoH

The electronic structures and the spectroscopic constants of the electronic ground 3Phi and low-lying 5Phi electronic excited states of the CoH molecule were studied by multireference single and double excitation configuration interaction (MR-SDCI)+Davidson's correction (Q) calculations and size-consistent multireference coupled pair approximation (MRCPA) calculations. Calculations were performed under Cinfinityv symmetry using Slater-type basis functions. The electronic ground state was confirmed to be the 3Phi state. It was found that at least four reference configurations were needed to describe the ground 3Phi state correctly at the MR-SDCI+Q level, while the 5Phi state can be described well by one reference configuration, namely, the Hartree-Fock configuration. Larger dynamical electron correlation for the low-spin 3Phi state than that for the high-spin 5Phi state is discussed. Spectroscopic constants, i.e., equilibrium bond lengths (re), harmonic frequency (omegae), and excitation energy, obtained by the MR-SDCI+Q method showed good correspondence with experimental values. MRCPA calculations gave a slightly shorter value for re than experimental values, but improved omegae and the excitation energy bringing them very close to experimental values.     

Ultrathin layer convolution

Ultrathin layers are defined as thin layers which are sufficiently thin that no concentration gradients are established within the layer on the time scale of a voltammetric measurement. Mathematically, ultrathin layers are characterized by ordinary differential equations in time. These equations are simpler to solve than the space and time dependent partial differential equations which describe all other electrode geometries. In this paper, a method is presented which capitalizes on the mathematical simplicity of the ultrathin layer to model any arbitrary, parameterizable electrode geometry. Laplace transforms are used to find an integral relationship between the current response of the modeled geometry and the ultrathin layer current. The integral relationship can be evaluated either analytically or numerically. Any voltammetric perturbation, under either Nernstian or mass transport-controlled conditions, can be modeled. The method is demonstrated for both planar and spherical electrodes. Cyclic voltammetric responses are modeled numerically and potential step responses are modeled analytically. It is also shown that for cyclic voltammetric perturbations, the current-voltage curves for the following systems have the same functional form. That is, the curves are identical within known multiplicative constants. The functionally equivalent responses are for (1) radial diffusion to a point electrode (the polarographic curve), (2) convective transport to a rotating disk, (3) the integral of the ultrathin layer response, and (4) the convolution or semi-integration of the response for linear diffusion to a planar electrode.     

Adsorption and Exchange Dynamics in Aging Hydroxyethylcellulose Layers on Silica

The adsorption kinetics of hydroxyethylcellulose (HEC) on silica and relaxations in adsorbed HEC layers were probed using total internal reflectance fluorescence and near-Brewster reflectivity. Like many random-coil polymers, HEC was found to adsorb at the transport-limited rate. Relaxations occurred at nearly constant interfacial mass when HEC layers were exposed to aqueous solvent, causing the subsequent exchange of chains between the layer and the free solution to become increasingly hindered. Eventually, on the time scale of a day, layers became immobilized and unable to accommodate chains from free solution. A continued fluorescence decay, beyond time scales that could be probed with self exchange, suggested further relaxations of the adsorbed HEC. The polydisperse HEC system (with an average molecular weight near 450,000) behaved qualitatively similar to molecular weight standard polyethylene oxide (PEO) layers on silica. For instance, relaxations in PEO layers occurred on a time scale of 10-20 h, like the HEC layers. Young layers of the latter, however, exhibited self-exchange kinetics that were an order of magnitude slower than PEO layers of similar age. This difference in adsorbed layer dynamics was attributed to HEC's stiffer backbone, compared with flexible PEO. Copyright 2000 Academic Press.     

Diarylamino groups as photostable auxofluors in 2-benzoxazolylfluorene, 2,5-diphenyloxazoles, 1,3,5-hexatrienes, 1,4-distyrylbenzenes,and 2,7-distyrylfluorenes

The relationship of structure to optical spectral properties was determined for five types of fluors in a search for an optimum-wavelength shifter to be used as part of the detection systems for high-energy particles from accelerators. In a search for photostable fluors to serve as waveshifters in plastic fibers it was found that the wavelengths of interest, absorption max 410 +/- 10 nm and fluorescence emission max 480 +/- 20 nm, along with other properties, such as high solubility and short fluorescence decay time, could be obtained from fluorophors composed of aromatic rings and vinyl groups only by using amino groups as auxochromes to give bathochromic shifts of wavelengths. Since primary, monoalkyl, and dialkylamino groups were not sufficiently photostable, a number of fluorophores bearing diarylamino groups were investigated. Syntheses of the fluors made use of the Buchwald amination, an improved version of the Emmons-Horner reaction, and other common reactions. The fluor types were the following: a 2-benzoxazolyl-7-(4-diarylamino)fluorene 7, 2-(4-cyanophenyl)-5-(4-aminophenyl)oxazoles 14 and 20, 1,3,5-hexatrienes 24a-d and 26a-c, 1,4-distyrylbenzenes 31d-g and 32a-e, and 2,7-distyrylfluorenes 40a,d-e. The unsymmetrical fluors 7, 14, and 20 were not as bright as the best hexatrienes, distyrylbenzenes, and distyrylfluorenes, which were all symmetrical. Where the 1,6-diaryl-1,3,5-hexatrienes 24a-d had high fluorescence quantum yield (Phi(f)), the 1,1,6,6-tetraryl-1,3,5-hexatrienes 26a-c had both lower epsilon and Phi(f). Where the 1,4-distyrylbenzenes 31d-g had high Phi(f), the 1,4-bis(2-phenylstyryl)benzenes 32a-e had Phi(f) = 0. Diarylamino groups as auxofluors conferred higher photochemical stability than dialkylamino groups on similar fluorophores. The 1,4-distyrylbenzenes 31d,e and the 2,7-distyrylfluorenes 40d,ehad the most desirable properties overall, which included fast decay times of 2 ns. Computer simulations predicted absorption and emission wavelengths fairly well, but were of little help for the prediction of brightness, stability, Phi(f), or decay time.     

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 1. Formation of the first layer

The formation and properties of adsorption layers of poly(dimethyldiallylammonium chloride) with different molecular masses on the surface of fused quartz are studied by the capillary electrokinetics method. It is shown that the value of ζ potential depends on the flow rate of liquid determined by the pressure drop. Such behavior can be explained by the deformation of the adsorption layer. At low rates of liquid flow, constant values of adsorption and time needed to achieve these values decrease for the samples of lower molecular masses, which is probably related to the more closely packed structure and, hence, to the lower deformability of the adsorption layers, as well as to the shortest times during which conformational rearrangements proceed in the layer. The time of conformational changes in the adsorption layer significantly exceeds the time of adsorption. The adsorption of cationic polyelectrolyte is irreversible. It is found that the compaction of adsorption layers increases with time; the rate of compaction of layers of a low-molecular-mass polyelectrolyte is higher and the layers of a high-molecular-mass polyelectrolyte retain the residual deformability even for six days. The measurements of the filtration of polyelectrolyte solutions through thin quartz capillaries allow the thickness of adsorption layers and their deformation under pressure to be estimated.     

Ground and lowest-lying electronic states of CoN. A multiconfigurational study

The lowest-lying X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II electronic states of CoN have been investigated at the ab initio MRCI and MS-CASPT2 levels, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the singlet states, the A1Phi and B1II states have been described for the first time. Potential energy curves, excitation energies, spectroscopic constants, and bonding character for all states are reported. Comparison with other early transition-metal nitrides (ScN, TiN, VN, and CrN), isoelectronic (NiC) and isovalent (RhN and IrN) species has been made, besides analyzing the B1II <=> X1+ electronic transition in terms of Franck-Condon factors, Einstein coefficients, and radiative lifetimes. At both levels of theory, the following energetic order has been obtained: X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II, with good agreement with experimental results. In contrast, previous DFT and MRCI calculations predicted the ground state to be the 5Delta state.     

Electrochemical properties of interface formed by interlaced layers of DNA- and lysozyme-coated single-walled carbon nanotubes

Multifunctional coatings were produced by the layer by layer assembly of single-walled carbon nanotubes (SWNT) dispersed in DNA and lysozyme (LSZ) on an insulating glass substrate. The electrochemical properties of these mechanically robust biocoatings were characterized for the first time using scanning electrochemical microscopy (SECM) and impedance spectroscopy (IS). SECM surface analysis demonstrated an increase in tip current with a corresponding increase in the number of oppositely polarized interlaced layers, indicating that subsequent layers were not electrically insulated from each other and a direct correlation exists between SECM feedback response and the number of layers. The rate of charge transport was also dependent on the chemical composition/polarity of the outermost surface layer. Coatings terminating in SWNT-DNA resulted in more positive feedback than those terminating in SWNT-LSZ. IS analysis demonstrated that the SWNT-DNA had a low charge transfer resistance in comparison with SWNT-LSZ, which is consistent with the results obtained by SECM. These results enable enhanced fundamental understanding and prediction of the electrical properties of SWNT-biopolymer layers with controlled interlaced polarities and orientation. Furthermore, these finding highlight the potential for SWNT-biopolymers in electronic and sensing applications.     

Effects of trimethylamine N-oxide (TMAO) and crowding agents on the stability of RNA hairpins

We study the effect of the osmolyte, Trimethylamine N-Oxide (TMAO), which accumulates in cells in response to osmotic stress, on the stability of RNA hairpins. All atom molecular dynamics (MD) simulations of a nucleotide and the 22-nucleotide RNA hairpin P5GA in an aqueous TMAO solution show that TMAO preferentially interacts with the base through the formation of a single hydrogen bond. To circumvent the difficulties of adequately sampling the conformational space of polynucleotides, we used coarse-grained models (including one that is inspired by the results of all-atom MD simulations of a single nucleotide) to probe the effects of osmoyltes on the stability of P5GA. If, as revealed by our MD simulations, the cosolute specifically interacts with only one base at a time, then we find practically no change in hairpin stability as measured by Delta T m = T m(Phi) - T m, where T m(Phi) and T m are the melting temperatures at volume fraction Phi of the osmolyte and Phi = 0, respectively. This finding is in qualitative agreement with recent experiments. If the interactions between the RNA and osmolytes are repulsive, which is appropriate for mimicking the effects of crowding, Delta T m can vary from 5 to 15 K depending on the size of the osmolyte and the nature of RNA-osmolyte interactions. Cosolutes that interact favorably with multiple bases simultaneously can stabilize the hairpin more than a crowding agent of the same size. The implications of our predictions for experiments are briefly outlined.     

Kinetics of phenol oxidation with ozone in a thin layer on a solid surface

Ozone has been reacted with phenol in thin supported layers, and the dynamics of this reaction has been investigated. The stoichiometry of this reaction coincides with the stoichiometry of the same reaction in solution. Specific reaction rate (β) has been determined for various phenol conversions. The effective rate constant of the reaction, estimated by extrapolating β to zero reaction time, is significantly higher than the rate constant of the reaction in solution. The reaction between ozone and phenol is diffusion-controlled. The reaction products form a barrier layer, which protects the deeper phenol layers against ozone. The barrier layer is as thick as 8–15 phenol monolayers.     

Optical behaviors of conjugated-chain compounds containing benzene and furan units

Thirteen conjugated-chain compounds which contain benzene and furan units were prepared, their optical behaviors, including UV-vis absorption coefficient (varepsilon), absorption wavelengths (lambdaa), fluorescence emission wavelengths (lambdae), and quantum yields (Phi) were measured. Meanwhile, their LUMO and HOMO energy were determined by cyclic voltammetry and their second-order polarizations (betaxxx) values were determined by solvatochromic method, respectively. The results showed that this kind of compounds possess a shorter lambdaa (320-365 nm) and performance a higher Phi values, especially for 2aa, 2ab, 2ac and 2bb, their Phi values are all more than 90%. These compounds, except 2db, showed a higher betaxxx values in DMSO, especially for 2dc (75.77x10(-30) m5 C-1) and 2dd (83.32x10(-30) m5 C-1), than that 10-methyl-acridone (6.578x10(-30) m5 C-1) or 10-benzylacridone (6.845x10(-30) m5 C-1) in DMSO did, and second harmonic generation value of 10-methylacridone and 10-benzylacridone in powder are, respectively, 1.381 and 1.861 times of that value of urea. The betaxxx values and Phi values determined for these compounds in this work were lower than these values which were desired in the original work, this phenomena was explained from their molecular structures. This work confirmed that as these compounds performance shorter lambdaa and higher Phi values, they could be good blue-color optical materials for some fields, such as OLED materials, two-photo absorption materials, fluorescent dyes.     

Photoinduced single- versus double-bond torsion in donor-acceptor-substituted trans-stilbenes

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), and fluorescence lifetimes of trans-4-(N-arylamino)-4'-cyanostilbenes (2H, 2Me, 2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4'-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4'-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4'-nitrostilbene (3MP), and three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) < 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) for judging the importance of a TICT state for trans-stilbenes is provided. Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4'-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS).     

Electrical properties of 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene and its application for organic light emitting diodes

We found that a phenylene ethynylene derivative, 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene (BPPB), provides very high photoluminescence efficiency both in solution (Phi(PL) = 95 +/- 3%) and thin films (Phi(PL) = 71 +/- 3%); further, we observed blue electroluminescence (EL) of lambda(EL(max)) approximately 470 and 510 nm with an external EL efficiency of eta(EL) approximately 0.53% and maximum luminance of approximately 70000 cd m(-2) at current density of approximately 2 A cm(-2) with BPPB as an emitter; also we identified that BPPB functions as a hole transport layer in organic light emitting diodes.     

聚酯低聚物和聚乙二醇醚对蒙脱土的插层与剥离作用

高分子基 /层状硅酸盐钠米复合材料的制备得益于自然界存在的一类层状硅酸盐 .它具有阳离子可交换性和在一些介质中的可膨润性 .在层状硅酸盐中蒙脱土是使用较普遍的一种 .它的晶胞系由两层Ｓｉ—Ｏ四面体中间夹一层Ａｌ—Ｏ(ＯＨ)八面体组成 .晶胞呈平行迭置 .层内有剩余电子而使其带负电荷 ,它与层间的自由阳离子Ｎａ+、Ｋ+、Ｃａ2 +、Ｍｇ2 +等达到电荷平衡 ,保持其整体的电中性 .利用蒙脱土的阳离子可交换特性 ,将有机离子插层剂或反应单体插入ＭＭＴ层间 ,使其层间距撑大 ,并增加其在反应体系中的膨润性 ,使ＭＭＴ在反应体系中剥离 ,…     

Probing adsorbed fibronectin layer structure by kinetic analysis of monoclonal antibody binding

Adsorbed protein layers are often away from equilibrium and thus exhibit history dependent structures. We use the kinetics of monoclonal antibody binding, as measured using optical waveguide lightmode spectroscopy (OWLS), to investigate the structure of adsorbed fibronectin (Fn) layers formed under different kinetic paths. For all of the layers investigated, we find no difference between the apparent adsorption rate constants of (i) monoclonal antibodies specific to Fn's cell binding site (alpha-Fn) and (ii) monoclonal antibodies specific to cytochrome c (alpha-CC, as a control), indicating initial adsorption of antibodies to be non-specific. For certain layers, the saturation density and the initial projected area per antibody differ significantly between alpha-Fn and alpha-CC, suggesting specific binding to follow the initial non-specific attachment. The fraction of antibodies binding specifically to the Fn layer, and the number of Fn binding sites per specific binding event, are estimated in terms of the difference in initial projected areas between alpha-Fn and alpha-CC. For a Fn layer formed at a bulk concentration of 2 microg/mL, we find a decrease in specific binding with an increase in Fn layer formation time, suggesting post-adsorption structural changes of a lower density adsorbed layer diminish binding site availability. Conversely, for a Fn layer formed at a bulk concentration of 40 microg/mL, we find an increase in specific binding with an increase in the aging time of the Fn layer, implying post-adsorption structural changes reveal binding sites for a higher density adsorbed layer.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Abstract

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.